Unstable immiscible fluid displacement in porous media affects geological carbon sequestration, enhanced oil recovery, and groundwater contamination by non-aqueous phase liquids. Characterization of immiscible displacement processes at the pore scale is important to better understand macroscopic processes at the continuum scale. A series of displacement experiments was conducted to investigate the impacts of viscous and capillary forces on displacement stability and fluid saturation distributions in a homogeneous water-wet pore network micromodel with precisely microfabricated pore structures. Displacements were studied using seven wetting non-wetting fluid pairs with viscosity ratios M (viscosity of the advancing non-wetting fluid divided by the viscosity of the displaced wetting fluid) ranging 4 orders of magnitude from log M = 1.95 to 1.88. The micromodel was initially saturated with either polyethylene glycol 200 (PEG200) or water as the wetting fluid, which was then displaced by a non-wetting alkane fluid under different flow rates. Capillary numbers (Ca) ranged over 4 orders of magnitude for the reported experiments, from log Ca = 5.88 to -1.02. Fluorescent microscopy was used to visualize displacement and measure non-wetting fluid saturations and interfacial area. In the experiments initially saturated with PEG200, a viscous wetting fluid, unstable displacement occurred by viscous fingering over the whole range of imposed capillary numbers. For the experiments initially saturated with water, unstable displacement occurred by capillary fingering at low capillary numbers. When the viscous forces were increased by increasing the injection rate, crossover into stable displacement was observed for the fluid pairs with log M > 0. For unstable displacement experiments applying the same capillary number for the seven fluid pairs, non-wetting fluid saturations were higher when capillary fingering was the dominant fingering process compared to viscous fingering. These experiments extend the fundamental work by Lenormand et al. (Lenormand, R; Touboul, E.; Zarcone, C. Numerical models and experiments on immiscible displacements in porous media. J. Fluid Mech. 1988, 189, 165-187) using precision-fabricated water-wet micromodels and enhanced image analysis of the saturation distributions. Our saturation distributions are consistent with other published experimental work and confirm the numerical results obtained by Lenormand et al.